Multiwavelength Study of Massive Galaxies at z~2. II. Widespread Compton-thick Active Galactic Nuclei and the Concurrent Growth of Black Holes and Bulges

November 2007 • 2007ApJ...670..173D

Authors • Daddi, E. • Alexander, D. M. • Dickinson, M. • Gilli, R. • Renzini, A. • Elbaz, D. • Cimatti, A. • Chary, R. • Frayer, D. • Bauer, F. E. • Brandt, W. N. • Giavalisco, M. • Grogin, N. A. • Huynh, M. • Kurk, J. • Mignoli, M. • Morrison, G. • Pope, A. • Ravindranath, S.

Abstract • Approximately 20%-30% of 1.4<~z<~2.5 galaxies with K_Vega<22 detected with Spitzer MIPS at 24 μm show excess mid-IR emission relative to that expected based on the rates of star formation measured from other multiwavelength data. These galaxies also display some near-IR excess in Spitzer IRAC data, with an SED peaking longward of 1.6 μm in the rest frame, indicating the presence of warm dust emission usually absent in star-forming galaxies. Stacking Chandra data for the mid-IR excess galaxies yields a significant hard X-ray detection at rest-frame energies >6.2 keV. The stacked X-ray spectrum rises steeply at >10 keV, suggesting that these sources host Compton-thick AGNs with column densities N_H>~10²⁴ cm^-2 and an average, unobscured X-ray luminosity L_2-8keV~(1-4)×10⁴³ ergs s^-1. Their sky density (~3200 deg^-2) and space density (~2.6×10^-4 Mpc^-3) are twice those of X-ray-detected AGNs at z~2, and much larger than those of previously known Compton-thick sources at similar redshifts. The mid-IR excess galaxies are part of the long sought after population of distant heavily obscured AGNs predicted by synthesis models of the X-ray background. The fraction of mid-IR excess objects increases with galaxy mass, reaching ~50%-60% for M~10¹¹ M_solar, an effect likely connected with downsizing in galaxy formation. The ratio of the inferred black hole growth rate from these Compton-thick sources to the global star formation rate at z=2 is similar to the mass ratio of black holes to stars in local spheroids, implying concurrent growth of both within the precursors of today's massive galaxies.

Links

• PREPRINT http://arxiv.org/abs/0705.2832
• ELECTR https://doi.org/10.1086%2F521820
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2007ApJ...670..173D
• PDF https://stacks.iop.org/0004-637X/670/173/pdf
• DATA https://cda.harvard.edu/chaser?obsid=441,580,581,582,957,966,967,1431,1671,1672,2232,2233,2234,2239,2312,2313,2344,2386,2405,2406,2409,2421,2423,3293,3294,3388,3389,3390,3391,3408,3409
• DATA https://irsa.ipac.caltech.edu/bibdata/2007/D/2007ApJ...670..173D.html